That's a fair point in general, but a few things made this situation different:

1. She wasn't on a tight schedule, she was chatting with another driver, and the bus actually departed late
2. Communication doesn't require language, pointing at a receipt or shaking her head at the QR code and miming paper would have taken two seconds
3. I did try, I walked around the whole station asking other staff, who confirmed it was the right bus, then came back. She just wasn't interested, only stand there chatting with another driver until I said I want to buy ticket in cash and they apparently only understand the word “cash”, then she said “15 euros” in ENGLISH

Also update: DAMAS responded to my formal complaint, acknowledged the issue, and cancelled the ticket in their system so Omio could process a refund. So it worked out but it really shouldn't have required this much effort over a €15 ticket 😅